Nearly one in four preschool-aged children in the US is overweight or obese. This problem is significant because obesity in young children is linked with immediate and long-term health consequences including diabetes, hypertension, heart disease and certain types of cancer. What is more, the health effects of obesity are disproportionately experienced among low-income and ethnic minority children. Given that children's emerging dietary, physical activity, screen-based and sleep behaviors are deeply rooted in the family, effectively engaging parents and family caregivers in childhood obesity prevention is crucial to its success. Yet family engagement has proven challenging and effective family interventions targeting young children remain elusive. In response, we have developed and tested a new approach to obesity prevention in families. Grounded in community-based participatory research (CBPR), our approach recognizes parents as family experts, engages them as co-researchers and embeds the resulting empowerment-focused intervention - Communities for Healthy Living (CHL) - into Head Start, a national system of care reaching over one million low-income families. Building on our successful pilot assessment of CHL, and in collaboration with our Head Start partners including 18 centers serving over 2000 children each year, we will expand our CBPR approach to engage parents of 3- to-5-year old children and Head Start staff to refine adapt CHL to a new setting. To ensure the timely translation of this work, we will test CHL effectiveness using a practical behavioral trial (PBT) and addressing the following specific aims: (1) utilize CBPR to adapt and refine CHL for implementation in a new setting; (2) test the efficacy of CHL using a group randomized controlled trial; and (3) examine the interplay between CHL and environmental factors, including the socioeconomic, food and physical activity environments, on CHL efficacy. Child outcomes, including Body Mass Index and four target health behaviors (diet, physical activity, screen time, and sleep), will be assessed for all children enrolled in the participating centers using Head Start's existing data system. Primary innovations of this study include the engagement of parents and staff in the participatory process, an intervention that deviates from the traditional model of nutrition and parenting education and addresses broader family realities, and the utilization of existing Head Start data systems to evaluate CHL, all of which support future dissemination and sustainability.